Intricate roles of spacers and stickers of Arg-rich C9ORF72 dipeptide repeat proteins; from toxicity to targeting to membraneless organelles

C9ORF72, one of the most common genes implicated in amyotrophic lateral sclerosis and frontotemporal dementia, induces neurodegeneration through various pathways. The most notable is interference through liquid-liquid phase separation (LLPS). LLPS is a biophysical phenomenon involved in many fundamental biological processes, such as the formation of membraneless organelles (MLOs), transcription, and nucleocytoplasmic transport. The Arg-rich dipeptide repeat proteins (R-DPRs) produced from the aberrant C9ORF72 gene are highly charged and are incorporated into the phase-separated MLOs, inhibiting their functions. However, the detailed molecular mechanism remains to be elucidated. Recently, we analyzed the structure-function relationship of R-DPRs and clarified the mechanism by which the sticker Arg and the spacer Pro/Gly regulate cytotoxicity and subcellular localization. Natural R-DPRs contribute to the localization of specific MLOs. In this review, we discuss the roles of the sticker and spacer of R-DPRs in the LLPS and how they regulate subcellular localization, protein-protein interaction, and neurotoxicity.